In general, a differential power amplifier is an amplifier that generates output in response to signals differentially inputted two input terminals and there are various differential power amplifiers.
FIG. 1 is an exemplary diagram of a differential amplifier according to the related art. The differential amplifier includes two transistors of a first transistor 1 and a second transistor 2.
A first signal is RFIN+ inputted to the gate of the first transistor 1 and a second signal RFIN− is inputted to the gate of the second transistor 2. The first signal and the second signal have opposite phases because it is a differential configuration. Accordingly, a third signal RFOUT+ outputted to a first output port 3 connected to the drain of the first transistor 1 and a fourth signal RFOUT− outputted to a second output port 4 connected to the drain of the second transistor 2 also have opposite phases.
When the signals inputted to the gates of the transistors are outputted through the drains, the phases are inverted and the magnitudes are amplified. Accordingly, in the third signal, the phase is opposite to that of the first signal inputted to the first transistor 1 and the magnitude is amplified further than that of the first signal. Accordingly, in the fourth signal, the phase is opposite to that of the second signal inputted to the second transistor 2 and the magnitude is amplified further than that of the second signal.
FIG. 2 is an exemplary diagram of a differential amplifier where mode injection is applied in FIG. 1. The configuration illustrated in FIG. 2 is obtained by applying mode injection to the configuration of the differential amplifier of FIG. 1. The power amplifier using mode injection illustrated in FIG. 2 additionally include a third transistor 5 and a fourth transistor 6 in comparison to the configuration illustrated in FIG. 1.
The drain of the third transistor 5 is connected with the drain of the first transistor 1 and the source of the third transistor 5 is connected with the source of the first transistor 1. The gate of the third transistor 5 is connected with the drain of the second transistor 2, so the signal outputted to the drain of the second transistor 2 is the input of the third transistor 5.
Similarly, the drain of the fourth transistor 6 is connected with the drain of the second transistor 2 and the source of the fourth transistor 6 is connected with the source of the second transistor 2. The gate of the fourth transistor 6 is connected with the drain of the first transistor 1, so the signal outputted to the drain of the first transistor 1 is the input of the fourth transistor 6.
As a result, in the power amplifier illustrated in FIG. 2, the first signal RFIN+ and the second signal RFIN− are the input and the fifth signal RFOUT+ and the sixth signal RFOUT− are the output. The components substantially in charge of amplifying are, similar to the configuration illustrated in FIG. 1, the first transistor 1 and the second transistor 2, and the third transistor 5 and the fourth transistor 6 are added for the amplification. Accordingly, the fifth signal RFOUT+ and the sixth signal RFOUT− can have magnitudes amplified further than the third signal and the fourth signal in the configuration illustrated in FIG. 1.
That is, the gates of the third transistor 5 and the fourth transistor 6 which are input are connected with the drains of the second transistor 2 and the first transistor 1, respectively. Accordingly, the signals activating the third transistor 5 and the fourth transistor 6 are not the first signal RFIN+ and the second signal RFIN−, but nodes in the amplifier (the drain of the second transistor 2 and the drain of the first transistor 1).
Accordingly, when mode injection is provided to a power amplifier, as in FIG. 2, since there are transistors that provide additional amplification such as the third transistor 5 and the fourth transistor 6 even if the magnitudes of the input first signal RFIN+ and second signal RFIN− are not increased, the amplifier has higher gain than that of the amplifier illustrated in FIG. 1.
The largest advantage of the amplifier using mode injection according to the related art is to be able to improve the gain. In contrast, the large problem is high possibility of oscillation due to positive feedback by the additional transistors 5 and 6 for mode injection. That is, there is a problem in that a fifth signal or a sixth signal is outputted, even though a first signal or a second signal is not inputted.
The problem is described hereafter with reference to FIG. 2. Even though the first signal RFIN+ and the second signal RFIN− are not applied to the transistors 1 and 2, respectively, the third transistor 5 receives a signal from the drain of the second transistor 6 and the signal is amplified at the drain of the third transistor 5. The amplified signal is inputted to the gate of the fourth transistor 6 and amplified again at the drain of the fourth transistor 6, and then the amplified signal is applied to the gate of the third transistor 5 and amplified at the drain of the third transistor 5 repeatedly. Accordingly, the amplifier using mode injection in FIG. 2 causes amplification itself even if first signal RFIN+ and the second signal RFIN− are not inputted from the outside, which is generally called oscillation. It means that the output of an amplifier does not amplify an input signal, but the amplifier is likely to operate itself.
Accordingly, it is not preferable to apply an amplifier using mode injection which has those characteristics to a specific wireless communication system. In the related art, a large transistor that functions as a switch was additionally disposed at a VDD or GND node in order to solve the problem. The additional transistor does not apply a first signal RFIN+ and a second signal RFIN− and disconnect the VDD or GND node from the amplifier, when the amplifier needs to be turned off in a wireless communication system. This allows for control of an output signal as an input signal, so the problem described above can be solved a little, but there is a defect that a transistor is additionally used. Further, the transistor that functions as a switch requires a large gate width in comparison to the transistors for amplifiers in order to minimize the ohmic loss. Accordingly, the additional transistor occupies a large area in an integrated circuit, and accordingly, the manufacturing cost of the integrated circuit increases.